In one type of optical system, a light beam from a scene is processed through a set of optical elements to image the light beam onto a detector. The optical elements may be refractive, reflective, or a combination of each type. The detector converts the light signal to an electrical signal for further processing.
It is sometimes desirable that the optical system have more than one field of view (i.e., more than one focal length). In an example of interest, a spacecraft carrying an optical surveillance system overflies the earth in a roughly elliptical high earth orbit. The altitude of the spacecraft above the surface of the earth varies according to its position in the orbit. In order to keep the magnification of the image of the optical system approximately constant regardless of the position of the spacecraft in its orbit, the field of view and the focal length of the optical system must be changed.
One approach to providing the optical system with a changeable field of view is to utilize a zoom telescope in the optical system. The positions of the lenses or mirrors that form the zoom telescope may be varied axially along the light path to change the magnification, field of view, and focal length of the zoom telescope. Other types of zoom lenses may also be used. However, in all cases the optical systems with the zoom telescope are mechanically complex due to the need to move several lens or mirror groups with respect to each other. Significant boresight and image quality errors may also result. Additionally, the weight and volume of the zoom telescopes tend to be high, which is a significant disadvantage in spacecraft and aircraft applications.
There is a need for an improved approach to providing multiple fields of view in a optical system such as that used in a spacecraft in high earth orbit. The present invention fulfills this need, and further provides related advantages.